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You need a "direct airside transit visa" (DATV) to connect at Heathrow, and visas are never issued at UK airports, ever.
While theoretically you could get through the international connections corridor at Heathrow undetected without a DATV, the check-in staff in Tehran is particularly responsible to make sure they don't transport you without the proper documentation. So if you don't have a DATV, you're not getting on the flight to London.
So, again, you need a DATV.